In vitro fertilization is a powerful and widely used technique for the treatment of infertility. In this procedure, human eggs are retrieved and mixed with sperm in a culture dish to allow fertilization. The embryos are then transferred to the uterus on day 2/3, when it has between 4 and 8 cells respectively or day 5 or 6 at blastocyst stage. This technique is used for women with, for example, damaged or absent Fallopian tubes, endometriosis, male factor infertility and unexplained infertility. However, the implantation rate varies between 5% and 30%.
Under in vivo conditions, the embryo reaches the uterus at a blastocyst stage of development. Accordingly, embryo coculture techniques, used successfully in animals, represent an effort to improve the culture media for embryos such that a greater proportion of embryos will reach the blastocyst stage for improving the implantation and pregnancy rates. In addition, if coculture results in a higher implantation rate, fewer embryos could be transferred at each cycle, resulting in a decreased incidence of multiple pregnancies. A variety of coculture techniques have been therefore investigated, involving the use of feeder cell layers derived from a range of tissues, including the use of human reproductive tissues (i.e., endometrium). However, no standardized method of coculture has emerged and the optimal system for preimplantation human embryo culture has not yet been determined.